1. Field of the Invention
The present invention relates to a light-emitting device using gallium nitride compound semiconductor whose luminous efficiency is improved. Especially, the present invention relates to the device which emits the ultraviolet ray.
2. Description of the Related Art
A conventional light-emitting device, which have layers made of gallium nitride compound semiconductor laminated on a substrate, is known to have the following structure. The device has a sapphire substrate, and on the sapphire substrate the following layers are formed sequentially: a buffer layer made of aluminum nitride (AlN); an n-cladding and/or an n-contact layer of high carrier concentration, which is made of a silicon (Si) doped GaN of n-type conduction; an emission layer having a multi quantum-well (MQW) structure, in which a barrier layer made of GaN and a well layer made of InGaN are laminated alternately; a p-cladding layer made of magnesium (Mg) doped AlGaN of p-type conduction; and a p-contact layer made of magnesium (Mg) doped GaN of p-type conduction.
And a conventional light-emitting device using gallium nitride compound semiconductor which emits the ultraviolet ray is known to have an emission layer made of InGaN or AlGaN. The device having an emission layer made of InGaN can obtain an ultraviolet ray having a wavelength of lower than 380 nm, which is emitted from band to band, when a composition ratio of indium (In) is less than 5.5%. The device having an emission layer made of AlGaN can obtain an ultraviolet ray having a wavelength of 380 nm, which is emitted by a pair of donor and acceptor, when a composition ratio of aluminum (Al) is about 16% and the emission layer is doped with zinc (Zn) and silicon (Si).
However, a problem persists in luminous efficiency. In the conventional light-emitting devices using gallium nitride compound semiconductor, conditions for emitting light are not always optimized. Therefore, further improvement has been required, as presently appreciated by the present inventors.